1. Technical Field of the Invention
The present invention relates generally to material testing and more particularly to a sensor for detecting crack lengths.
2. Discussion of the Related Art
Structural defects such as flaws and cracks are formed in materials during the manufacturing process or during subsequent loading. These cracks propagate due to applied loads and can result in catastrophic failure. Two of the broad objectives of improved design methodology are to select tough materials that resist crack formation and crack propagation and to monitor the growth of the crack so that repair or replacement can be accomplished at appropriate intervals. Monitoring may be accomplished by actual measurement or by employing appropriate models.
Techniques for the detection and measurement of cracks depend upon several factors such as the nature of the material, crack size, crack location and the nature of the loading, i.e., static or dynamic loading. Visual inspection, either naked or aided by dye penetrant or magnetic particles; reduction in stiffness; increase in damping; x-ray radiography; holographic interferometry; ultrasonics; acoustic emission; eddy current; potential drop and resonant frequency measurements are some of the methods which have been developed for assessing the crack damage in components and structures. For applications where cracks are well defined and are present on the surface of the component or structure, special gages have been developed. These gages are bondable and give an electrical signal as the output. A crack detection coating, consisting of an epoxy base layer matrix containing micro-capsules filled with an electrically conductive emulsion and a top layer of silver conductive paint, has been marketed under the name "CDC-2" by B.L.H. Electronics of Waltham, Mass. A crack in the base structure propagates into the epoxy layer, rupturing the micro-capsules and causing conduction between the metallic structure and the conducting paint. Various crack detection gages and crack propagation gages incorporating thin metallic filaments are also commercially available from the Micro-Measurements Division of the Measurements Group, Inc. of Raleigh, N.C. A crack in the base structure, on which the gages are bonded, ruptures the gage filaments, resulting in an increase in the total resistance. In a variation of this concept, the crack propagation gage is deposited on the specimen by a silkscreen process using a silver-epoxy resin mixture.
The output from gages that consist of filaments or strands is discontinuous and the resolution depends upon the number of lines per unit length. To overcome this limitation, a bondable foil gage that produces a continuous DC output voltage proportional to the crack length through an indirect potential drop measurement technique has been developed and marketed by the Hartrun Corporation of St. Augustine, Fla. under the name "Krak-gage". The gage, of course, has to be used with the accompanying instrumentation. In the DC potential drop method, a constant direct current is passed through the specimen if it is conducting or through a thin conducting gage bonded to the specimen. The process is usually monitored by measuring the potential difference between two probes placed on either side of the crack. The technique requires that the calibration relationship between crack length and measured potential be determined either by experimental or theoretical means.
Thus, the bondable filamentary gages that do not require special instrumentation do not give a continuous output and the bondable foil gage that gives a continuous output requires special instrumentation. The direct bonding of nonmetallic resistor elements, in the form of a carbon coating, to a structure on which the strain has to be measured, is attributed to A. Bloch as early as 1935. Later this concept was utilized in fabricating gages, with a resistance varying between 15K.OMEGA. and 35K.OMEGA., which could be bonded to a structure for strain measurement. More recently, several types of conducting particle impregnated polymers have been investigated for possible applications as fatigue damage indicators, strain gages and overload indicators, as indicated in the Dally et al reference discussed below.